Increasing Man-Made Air Pollution Reduces Rainfall in Southern West Africa

Southern West Africa has one of the fastest growing populations worldwide. A decrease in precipitation in this region could severely affect water supply, and food and energy production for more than a hundred million people. Interannual to decadal variability in rainfall has been shown to be controlled by changes in sea-surface temperatures in the tropical Atlantic and Pacific Oceans. Precipitation also shows a strong seasonality associated with the West African monsoon. The occurrence and amount of rainfall events depend on surface solar heating and the organization of thunderstorms to large complexes.Urbanization and strong population growth in southern West Africa during the past decades have caused a marked increase in anthropogenic emissions. It is unclear to what extent increased air pollution has reduction incoming sunlight and whether this has led to changes in precipitation.

Here we show for the first time that a rainfall suppressing impact of the increased concentration of aerosol particles can indeed be separated from other climatic factors. The impacts are clearest for the little dry season (July–mid-September) and the following rainy season (until end of October) and vary spatially. Past research on this question has been hampered by the absence of long-term aerosol records of sufficient quality. Here we use the indirect evidence from observations of precipitation (CHIRPS and station data), radiation and clouds (CM-SAF satellite and station data) and visibility (station data), together with their spatio-temporal variations, to detect the pollution impact.

Removing trends related to large-scale climatic changes (visible in indices such as ATL3 and NINO34) from observed rainfall time series (1983–2015) allows us to quantify the residuum rainfall trend, which we assume to be due to the aerosol effect. In both seasons this residuum trend reaches about 0.6% per year. We corroborate the relation to aerosol by comparing observed radiation, cloud and horizontal visibility data sets. The observed reduction in incoming solar radiation over the past 35 years cannot fully be explained with increased cloud cover. At the same time, visibility observations over land and MODIS AOD data over the ocean both indicate increasing aerosol concentrations. The increase in both clouds and aerosol reduces surface solar heating and therefore stabilizes the boundary layer, which causes the observed rainfall reduction.

Policymakers in Africa are advised to prevent a further increase in air pollution, as this may exacerbate rainfall suppression and extend effects spatially and seasonally, as has already been shown for regions in southern Africa.